1. Field of the Invention
The present invention relates to a method for carrying out vapor-phase catalytic ammoxidation in a fluidized-bed reactor, and provides an economically advantageous method for producing, from a starting material to be ammoxidized (hereinafter referred to as a starting hydrocarbon or the like), a corresponding nitrile in an increased yield.
2. Related Art
Ammoxidation in which a hydrocarbon or the like is brought into contact with ammonia and oxygen in the presence of a catalyst to produce a nitrile by one reaction step is very useful from the industrial point of view. It is well known that, when methanol is used as the starting hydrocarbon or the like, hydrocyanic acid is produced (therefore, in the present invention, "starting materials to be ammoxidized" are not limited to hydrocarbons, and "nitriles" also include HCN); and that, when propylene, isobutylene or 2,6-dichlorotoluene is used, acrylonitrile, methacrylonitrile or 2,6-dichlorobenzo-nitrile is produced, respectively. These nitriles are used in large quantities as starting materials for polymeric compounds such as resins and fibers, and for a variety of chemicals.
In general, the optimum temperature range for carrying out the ammoxidation reaction of such a hydrocarbon or the like is narrow, and, in addition, the amount of exothermic energy generated by the reaction is large. Therefore, a fluidized-bed reactor is often used for this ammoxidation reaction because it is excellent in temperature controllability and because it can treat a high-concentration starting gas, and thus can attain high productivity. However, in a conventional fluidized-bed reactor, a starting gas to be fed to the reactor passes through a catalyst bed as a large number of bubbles, so that contact between the starting gas and the catalyst tends to be insufficient. On the other hand, a fluidized-bed catalyst is characterized in that the mixing of the catalyst particles, including the mixing of the catalyst particles in an upward stream with those in a downward stream can be successfully attained. However, due to this characteristic feature, the mixing of an upward gas stream with a downward gas stream, that is, back mixing tends to occur in the catalyst bed.
For the above-described reasons, when a conventional fluidized-bed reactor is used, the yield of a nitrile tends to be low, as compared with a case where a fixed-bed reactor is used. In order to solve this problem residing in fluidized-bed reaction, many proposals have been made so far. These proposals can be classified broadly into the following three types of methods.
Methods classified in the first group are such that shaped articles made out of wire-netting, screens, grids, perforated plates, horizontal plates, vertical pipes or the like are laid in a catalyst bed as obstacles to prevent the coalescence or growth of bubbles so as to promote the division of the bubbles, or to control the state of the mixing of the catalyst particles, thereby improving contact between the starting gas and the catalyst (the methods described in, for example, Japanese Patent Publications No. 2533/1965, No. 28491/1969, No. 531/1973 and No. 38428/1983 (the specification of U.S. Pat. No. 4,082,786), the specification of U.S. Pat. No. 3,783,528, etc.). In these methods, it seems that contact between a starting gas and a catalyst is improved when the obstacles are laid densely. However, such obstacles are not practical because the construction for laying them is complicated. Moreover, they excessively prevent the mixing of catalyst particles, so that the characteristics of the fluidized bed such that e.g., the temperature controllability is excellent, and the temperature of the catalyst bed becomes uniform would not be fully utilized; and, at the same time, the catalyst in the reactor is distributed unevenly in terms of space and time, so that it becomes difficult to carry out the reaction stably and continuously. On the other hand, when the obstacles are laid so that relatively good temperature controllability can be obtained, the contact between the reactant gas and the catalyst would not be fully improved.
Methods classified in the second group are to control the distribution of the concentration of a starting gas in a reactor in order to increase the rate of the utilization of the starting gas in the reaction. In these methods, a starting material is fed through two feed openings separately provided at two different points; or, after a starting hydrocarbon or the like is thoroughly mixed with ammonia and oxygen, the mixture is brought into substantial contact with a catalyst. These methods are described, for instance, in Japanese Patent Publication No. 41369/1970 (the specification of U.S. Pat. No. 3,546,268), Japanese Patent Laid-Open Publications No. 9751/1982, No. 258/1990 (the specification of U.S. Pat. No. 4,801,731) and No. 157355/1991. However, these methods have been proposed not for improving the efficiency of contact between a reactant gas and a catalyst, which is a fundamental problem confronting fluidized beds, but for improving the state of the mixing of a starting gas. Although the starting gas is maintained, for a short time after the feeding thereof, in a state which can meet the object, these methods would not solve the problem that a catalyst and a starting hydrocarbon or the like cannot be brought into intimate contact due to the growth or enlargement of bubbles generated while the starting gas is passing through the catalyst bed, and would not fully prevent the cause of uneven distribution of residence time due to the back mixing of the gas. These methods are thus unsatisfactory as a means for improving contact between a gas and catalyst particles.
Methods classified in the third group utilize a fluidized state which is essentially different from that in a conventional fluidized bed. These methods are such that a fluidized bed is formed by a large amount of catalyst particles which are transported and accompanied by a high-velocity gas stream. The solid matter density of this fluidized bed is relatively low as compared with that of a conventional fluidized bed, and the flow of the gas and that of the catalyst are similar to piston flow. Incidentally, a fluidized bed in such a state was termed "fast fluidization" by Joseph Yerushalmi et al. ("Industrial and Engineering Chemistry Process Design Development", Vol. 15, No. 1, pp. 47-53 (1976)).
As a technique utilizing this high-velocity fluidized bed, there has been known a method described in Japanese Patent Laid-Open Publication No. 144528/1978 (the specification of U.S. Pat. No. 4,102,914). This method is characterized in that reaction is carried out by using a solid matter density of approximately 16 to 240 kg/m.sup.3 and a gas velocity of approximately 1.5 to 7.5 m/s, the solid matter density being low and the gas velocity being high as compared with those in a conventional fluidized bed. However, although this method is advantageous in that the productivity per sectional area of a reactor used is high due to high gas velocity, it seems to have the following problem: in order to obtain a desired nitrile in high yield, an extremely long reaction zone is needed; to attain this, it is necessary to considerably increase the height of the reactor when the reactor is in the shape of vertical cylinder, which is a shape common to conventional reactors, so that the construction cost is increased. In order to avoid this problem, the reactor is formed to have a coil-like shape.
However, in a method using this coil-shaped reactor, since centrifugal force acts on catalyst particles which are passing through the coil, a gas and the catalyst particles are unevenly distributed, and contact between them becomes poor. Thus, the effect of improving the contact between the gas and the catalyst particles tends to be insufficient. Further, another problem residing in this method is as follows: if the catalyst is not separated and removed, in a large amount, from the reaction product immediately after the conversion of the starting hydrocarbon or the like reaches an optimum value, the reaction proceeds excessively, and a nitrile is produced in a decreased yield. Furthermore, a separator such as a cyclone is usually employed in this method as a catalyst separator, and, since the amount of the gas and that of the catalyst are large, the cost required for manufacturing a cyclone which can meet this condition, i.e., a cyclone which is large in size, has high collection efficiency, and hardly abrades or powderizes the catalyst, is high. In addition, in this method, it is, in general, necessary to lay a pipe which is useful for transferring the catalyst to the outside of the reactor system, in order to return the catalyst separated from the gas to the reactor again. Moreover, it is necessary to control the pressure balancing so that the reactant gas will not back-flow in this pipe.
Thus, the method for improving contact between a gas and catalyst particles by utilizing high-speed fluidization has such problems that an increased cost is required for plant construction, that the operation is complicated and that the effect for improving contact between a gas and catalyst particles would not be sufficiently obtained depending upon the type of the installation of the plant.
Therefore, expected effects would not be fully obtained.